1. Field of the Invention
The invention relates to a process for isolating highly pure 2-methoxypropene from crude products or other mixtures which comprise 2,2-dimethoxypropane, 2-methoxypropene, methanol and acetone, with or without other carbonyl compounds, which are obtained, for example, in the synthesis of 2-methoxypropene.
2. Description of the Background
2-Methoxypropene is a valuable intermediate which is required in high purity for the synthesis of active pharmaceutical ingredients. For use in the pharmaceuticals sector, purities of >99.0% are required and limiting values of impurities such as carbonyl compounds of max. 0.2%, especially of max. 0.1%, have to be complied with.
2-Methoxypropene can be prepared particularly advantageously by pyrolyzing 2,2-dimethoxypropane to eliminate methanol and form a crude mixture of 2-methoxypropene, methanol and acetone, and also a series of further impurities. The preparation may be performed either batchwise or continuously, by contacting the 2,2-dimethoxypropane reactant with a catalyst which is present, for example, as a fixed bed or as a contact zone. In the case of the continuous method, the crude product mixture is removed from the reaction zone simultaneously with the reactant feed and the reaction. Preparation processes for 2-methoxypropene via pyrolysis of 2,2-dimethoxypropane are described, for example, in EP 0 703 211 or in EP 0 776 879, WO 2001/096269 or Tetrahedron Letters 23 (6), 631–634, 1982.
Completely different synthetic routes for the preparation of 2-methoxypropene start from 1-propyne or propene and are described, for example, in DE 102 33 231 or in J. Am. Chem. Soc 1967, 89(18), 4684–7.
However, owing to the impurities present therein, especially as a result of its content of carbonyl compounds, e.g. acetone, the 2-methoxypropene obtained by all of these routes does not in any way satisfy the purity requirements for pharmaceutical application. Simple distillative removal is virtually impossible with acceptable technical cost and inconvenience, since binary, ternary or multiple azeotropes form [Beregovykh, V. V.; Andrianove, O. N.; Babich, S. V.; Khimiko-Farnatsevticheskij Zhumal (1983), 17(4), 454–9] and extractive removal of carbonyl compounds is not very effective [Agre, B. A. et al. Vses. Nauchno-Issled. Inst. Org. Sint. Moscow Neftepererabotka i Neftechimiya (Moscow) 1983, (1), 37–38].
According to the prior art, a series of chemical purification processes for aftertreating the crude mixtures is known, in which traces of carbonyl compounds can be removed from the synthesis.
For instance, the patent DE 33 23 823 describes a process for removing impurities (aldehydes and ketones) by the aftertreatment with NaBH4. In order to minimize 0.1–0.2% of impurities to a value of <0.1%, amounts of approx. 10% by weight of NaBH4 and 30–40% by weight of NaOH are needed, which correspondingly increases the complexity of purification and makes the product more expensive.
In a further publication, carbonylic impurities are removed by adding diols which are derivatized as ketals or acetals and can then be removed from the products of value by distillation, as mentioned, for example, in the patent U.S. Pat. No. 3,578,568.
The patent U.S. Pat. No. 4,012,289 describes a further purification process for removing acetone traces by adding sulfolane.
In other patents which describe the purification of acetone-containing product mixtures, acetone traces are removed by air oxidation in the presence of a CuNi catalyst on alumina support (SU 728 902) and (SU 662 585).
A further way of removing acetone traces is described in the patent U.S. Pat. No. 5,352,807 by treating with activated carbon as an adsorbent. Using supercritical CO2, acetone is removed extractively as an impurity according to patent RU 2 110 523. Further processes describe the removal of small amounts of acetone from distillation products by treating with hydroxylamine (patent CS 109 181 and Levadie, Benjamin; Mac Askill, Stephen; Analytical Chemistry 1976, 48(11), 1656 ff.).
U.S. Pat. No. 5,271,835 describes a process for removing sulfuric impurities from light oil streams by treating with alkanolamines, for example methyldiethanolamine, diethanolamine and monoethanolamine. However, traces of polar components such as acetone are removed there mainly on a solid adsorbent which subsequently has to be regenerated again in a costly and inconvenient manner.
According to the patent U.S. Pat. No. 3,607,003, acetone can be removed from hydrogen gas streams which comprise acetone as an impurity using a scrubber, but one which contains an aqueous monoethanolamine solution and is operated at room temperature.
The laid-open specification DE 11 34 076 describes the formation of Schiff bases from carbonyl compounds and primary amines including ethanolamine, by, according to the invention, transferring the imino function from a Schiff base of a preferably low molecular weight carbonyl compound to the carbonyl function of a second carbonyl compound. The target product, the Schiff base of the second carbonyl compound, is removed from the low molecular weight carbonyl compound released. This is effected by crystallizing out and optionally filtration in the case of a sparingly soluble reaction product, or distilling off the newly formed Schiff base, or preferably by distilling off the low molecular weight carbonyl compound. Full retention of the carbonyl compound in the bottoms of the distillation is not described here.
All prior art processes have either the disadvantage that they are technically very complicated, as, for example, in the abovementioned patent DE 11 34 076, or that they do not satisfy the degree of purification required or the efficiency of the removal of carbonyl compounds for a pharmaceutical application of 2-methoxypropene, for which it is important not to exceed an upper limit of max. 0.2%, but preferably max. 0.1%, of acetone.
It is therefore an object of the invention to provide a process, very simple to carry out on the industrial scale, for isolating 2-methoxypropene from 2-methoxypropene-containing mixtures which enables very good yields and provides a highly pure product which may contain a maximum of 0.2%, but preferably max. 0.1%, of carbonyl compound, especially acetone. It should be possible in particular to use crude products or mixtures as the starting material which, in addition to 2,2-dimethoxypropane, may contain methanol, acetone and also further carbonyl compounds as impurities. In particular, it should be possible to use reaction effluents of the 2-methoxypropene synthesis. The process should as far as possible be performable without great additional apparatus complexity and especially the disadvantages of the conventional processes should be avoided.